I conjecture, therefore, that the moment in which intelligence awakens to activity is marked by the infant's first smile.
By this ineffable expression of its joy, the infant seems to hail the light of the day which is dawning upon him. His reasonable soul rejoices in the truth which it recovers, and springs forward, as it were, to clasp it. How great, how solemn a moment to the human soul, must be the first act of its intelligence, the sense of a new and boundless life, the discovery of its own immortality! Is it possible that an event so stupendous and so startling to the infant, though the adult can form no idea of it, should not be manifested externally by signs of exuberant joy? You are right, then, O mothers, who watch so eagerly for your infant's first smile, who try to induce it, who welcome it with such trembling joy in every fibre of your being. You alone are the true interpreters of those first utterances of infancy which, in the shape of a smile, break from the lips and the eyes and the whole countenance of the little intelligent being; you alone understand its mystery; you understand that from that hour he knows you and speaks to you; and you, the first object of human intelligence, you alone know how to answer this language of love, and to make yourselves the image and type of the truth which is intelligible, and which shines by its own light.
Antonio Rosmini, The Ruling Principle of Method Applied to Education, pp. 60-61. Rosmini's idea is that infants have first a purely sensory period, beginning in the mother's womb, in which they are, so to speak, swept on by the world, and then, stimulated to have a sense of wanting other sensations, they begin to become mental agents -- and Rosmini's proposal is that the sign that this has begun is the infant's first clear expression of delight, which establishes that they have attended to and recognized something so as to be delightedly startled at it. From this point on, with this smile, he thinks, the infant's intellectual life begins its steady development.